Extension Cost Calculator: Get an Instant Estimate for Your Home in 2026
Adding an extension to your home is one of the most impactful improvements you can make — but understanding the true cost before you start is essential. In 2026, the average cost of a UK home extension ranges from £1,800 to £2,800 per square metre for a general single-storey rear extension, according to data from the Federation of Master Builders (FMB) and Checkatrade. For a typical 20 m² extension, that means a build cost of approximately £36,000 to £56,000, before you factor in professional fees, planning costs, and contingency.
At Suddeco Homes, we believe every homeowner deserves clarity on costs before committing to an architect or builder. Our free render tool gives you an instant visual estimate based on your address — showing what your neighbours have built, what it cost, and what your property could achieve. This guide breaks down every cost element you need to budget for, with real 2026 figures sourced from industry leaders.
Table of Contents
- What Is an Extension Cost Calculator?
- Extension Cost Breakdown Table (2026)
- Location-Specific Pricing: London vs. South East vs. North
- What Affects the Cost of an Extension?
- How to Save Money on Your Extension
- Professional Fees and Hidden Costs
- How Suddeco Helps You Budget Accurately
- Frequently Asked Questions
What Is an Extension Cost Calculator?
An extension cost calculator is a tool that estimates the total cost of building an extension based on your location, project size, and specification level. Unlike rough online estimates that apply a generic rate, a proper calculator accounts for regional labour costs, material choices, and the complexity of your specific project.
The most reliable way to calculate extension costs is to use a per-square-metre rate combined with location adjustments. This approach is used by the HomeOwners Alliance and RICS as the industry standard for initial budgeting. At Suddeco, our free render tool goes further by analysing nearby planning approvals to show you what has actually been built and approved on your street — giving you a real-world baseline rather than a guess.
Extension Cost Breakdown Table (2026)
The following table shows the per-m² cost ranges for the most common extension types in the UK. Figures are based on 2026 data from Checkatrade, the FMB, and HomeOwners Alliance, and include labour, materials, and basic finishes. They exclude professional fees, VAT, and contingency.
| Extension Type | Low Cost (per m²) | Average Cost (per m²) | High Cost (per m²) | Typical Total (20 m²) |
|---|---|---|---|---|
| Single-storey rear | £1,800 | £2,300 | £2,800 | £36,000–£56,000 |
| Double-storey rear | £1,800 | £2,150 | £2,500 | £72,000–£100,000 |
| Kitchen extension | £2,000 | £2,500 | £3,000 | £40,000–£60,000 |
| Side return | £2,000 | £2,500 | £3,000 | £30,000–£45,000 |
| Wrap-around | £2,000 | £2,500 | £3,000 | £50,000–£75,000 |
| Loft conversion | £1,500 | £2,000 | £2,500 | £30,000–£50,000 |
| Garage conversion | £800 | £1,150 | £1,500 | £12,000–£22,500 |
| Conservatory | £1,200 | £1,600 | £2,000 | £18,000–£30,000 |
| Orangery | £2,000 | £2,500 | £3,000 | £30,000–£45,000 |
Sources: Checkatrade (2026), Federation of Master Builders (FMB), HomeOwners Alliance. Costs are for construction only and exclude VAT, professional fees, and contingency.
Key Cost Drivers Within These Ranges
The gap between low and high costs depends on several factors:
- Specification level: Basic finishes (matt paint, standard kitchen, laminate flooring) sit at the lower end. High-end finishes (bespoke kitchen, underfloor heating, bi-fold doors, oak flooring) push costs toward the upper limit.
- Structural complexity: Removing load-bearing walls, adding steel beams, or excavating for foundations increases costs.
- Glazing and doors: Large sliding or bi-fold door systems can add £5,000–£15,000 to the total.
- Kitchen and bathroom fit-outs: These are the most expensive rooms to fit. A full kitchen installation can add £10,000–£25,000 on top of the build cost.
Location-Specific Pricing: London vs. South East vs. North
Where you live in the UK has a significant impact on your extension cost. Labour rates, material delivery charges, and local demand all vary by region.
London Premium: +30–50%
London remains the most expensive region for construction. The high cost of living, congestion charges, restricted access, and elevated labour rates push per-m² costs well above the national average. A single-storey rear extension in London typically costs £2,340–£4,200 per m², with premium postcodes in Kensington, Chelsea, and Westminster at the top of that range. A 20 m² extension in London could cost anywhere from £47,000 to £84,000 for construction alone.
South East Premium: +20–30%
Areas such as Surrey, Berkshire, Buckinghamshire, and Hertfordshire experience a moderate premium due to proximity to London and high property values. Expect to pay £2,160–£3,640 per m² for a general extension. A 20 m² extension in the South East would typically range from £43,000 to £73,000.
North and Midlands: -10–20% Below National Average
Regions including Yorkshire, the North East, North West, and the Midlands benefit from lower labour costs and less congested sites. Per-m² costs here are typically £1,440–£2,240, meaning a 20 m² extension could cost £29,000 to £45,000. Scotland and Wales are similarly positioned, with some rural areas offering even lower rates due to reduced demand.
| Region | Typical Premium | Single-Storey Cost (per m²) | 20 m² Extension Total |
|---|---|---|---|
| London | +30–50% | £2,340–£4,200 | £47,000–£84,000 |
| South East | +20–30% | £2,160–£3,640 | £43,000–£73,000 |
| South West | +10–15% | £1,980–£3,220 | £40,000–£64,000 |
| Midlands | -10% | £1,620–£2,520 | £32,000–£50,000 |
| North England | -15% | £1,530–£2,380 | £31,000–£48,000 |
| Scotland | -15–20% | £1,440–£2,240 | £29,000–£45,000 |
| Wales | -10–15% | £1,530–£2,380 | £31,000–£48,000 |
Source: Checkatrade regional cost data 2026; FMB regional labour rate survey.
What Affects the Cost of an Extension?
Understanding the variables that drive cost helps you control your budget and avoid surprises. The main factors are:
1. Size and Shape
Larger extensions cost more in total but often have a lower per-m² rate due to economies of scale. A 40 m² extension will not cost exactly double a 20 m² extension because fixed costs (foundations, scaffolding, site setup) are spread over a larger area. Complex shapes with multiple corners or irregular footprints increase labour and material waste.
2. Materials and Finishes
The difference between a budget and luxury specification can be £500–£1,000 per m². Key material choices include:
- Foundations: Standard trench fill vs. piled foundations (required on poor soil or sloping sites) — difference of £3,000–£8,000.
- Roof: Flat roof with EPDM membrane vs. pitched roof with slate tiles — difference of £2,000–£5,000 for a 20 m² extension.
- Glazing: Standard uPVC windows vs. aluminium bi-fold doors — difference of £3,000–£10,000.
- Flooring: Laminate at £15–£30/m² vs. engineered oak at £60–£120/m².
- Kitchen: Off-the-shelf kitchen at £5,000–£8,000 vs. bespoke at £20,000–£40,000.
3. Structural Complexity
Removing internal walls to create open-plan living requires steel beams and structural calculations. This typically adds £2,000–£5,000 to the project. If your property is on a slope, has poor soil conditions, or requires underpinning, foundation costs can increase by £5,000–£15,000.
4. Access and Site Conditions
Restricted access (narrow alleyways, no rear access, terraced houses) increases labour costs because materials must be carried through the house or craned over. A typical access restriction adds £1,000–£3,000 to the project. Working in a conservation area or on a listed building also increases costs due to additional planning requirements and material specifications.
5. Services and Utilities
Moving drainage, gas, electricity, or water supplies can add £1,000–£5,000 depending on the complexity. Connecting a new kitchen to existing services is usually straightforward, but relocating a boiler or adding a new bathroom may require significant plumbing and electrical work.
Professional Fees and Hidden Costs
The construction cost is only part of the picture. Budget for these additional expenses:
| Cost Item | Typical Range | Notes |
|---|---|---|
| Architect fees | 5–10% of build cost | £3,750–£7,500 for a £75k extension |
| Structural engineer | £500–£2,000 | Required for steel beam calculations |
| Planning permission (householder) | £206 | Standard application fee to local council |
| Planning permission (full) | £462 | For more complex or non-householder applications |
| Building regulations | £200–£1,000 | Inspection and approval fees |
| Party wall agreement | £700–£2,000 | Required if building on or near a boundary with a neighbour |
| Site survey | £400–£1,000 | Topographical or measured survey |
| Contingency | 10–15% of build cost | Essential buffer for unforeseen issues |
| VAT | 20% | Applies to most builder invoices unless VAT-registered DIY |
Sources: Planning Portal (2026 fee schedule), RICS fee guidelines, HomeOwners Alliance.
Total Budget Example: 20 m² Single-Storey Extension in the Midlands
| Item | Cost |
|---|---|
| Construction (20 m² at £2,000/m²) | £40,000 |
| Architect (7.5% of build cost) | £3,000 |
| Structural engineer | £1,000 |
| Planning permission | £206 |
| Building regulations | £500 |
| Party wall agreement | £1,000 |
| Contingency (12.5%) | £5,000 |
| Subtotal | £50,706 |
| VAT (20%) | £10,141 |
| Total Project Cost | £60,847 |
This illustrates why a simple per-m² rate can be misleading. The full project cost is typically 25–40% higher than the construction cost alone once fees, contingency, and VAT are included.
How to Save Money on Your Extension
1. Get a Free Render Before You Commit
The biggest cost-saving move is knowing what is possible before you spend money on architects or builders. Our free render tool shows you what has been approved near your address, what it looks like, and what it cost. This gives you a realistic benchmark and prevents over-specification.
2. Use Standard Dimensions
Designing your extension to standard material sizes reduces waste and labour. For example, using standard 2.4 m stud wall heights, 1.2 m plasterboard widths, and standard door and window sizes can save £1,000–£3,000 on materials and cutting time.
3. Keep the Design Simple
Rectangular shapes with flat roofs are cheaper than complex forms with multiple roof pitches. Avoid unnecessary steelwork by keeping internal walls where possible. A simple design can reduce costs by 10–20% without compromising functionality.
4. Choose Off-the-Shelf Materials
Bespoke windows, doors, and kitchens look beautiful but carry a premium of 30–50% over standard products. High-quality off-the-shelf options from reputable suppliers offer excellent aesthetics at a fraction of the cost.
5. Consider a Partial DIY Approach
If you are competent, undertaking demolition, basic landscaping, or decoration yourself can save £2,000–£5,000. Never attempt structural, electrical, or gas work yourself — these must be done by qualified professionals and signed off by building control.
6. Time Your Build Carefully
Builders are often quieter in January and February, and some offer discounted rates during off-peak months. Avoid starting in the run-up to Christmas or during school summer holidays when demand is highest.
7. Get Multiple Quotes
Always obtain at least three detailed quotes from builders. Checkatrade recommends comparing quotes on a like-for-like basis — ensure each builder is quoting for the same specification, finishes, and scope. The cheapest quote is not always the best; look for transparency, references, and clear payment terms.
How Suddeco Helps You Budget Accurately
Traditional cost estimation relies on rough rules of thumb that rarely match your specific situation. Suddeco Homes uses planning data from your local area to show you what has actually been built, approved, and — where available — sold for.
Free Render: Your Starting Point
Enter your address on our free render page and receive an instant visualisation of what your property could become. We analyse nearby approved planning applications to show you real precedents, including extension types, sizes, and estimated costs. This is completely free and takes less than a minute.
Concept Package: £495–£995
Once you have a direction, our Concept Package delivers three photorealistic renders, full floor plans, and a detailed cost estimate within 48 hours. You can revise the design once before moving forward. This gives you a presentation-quality vision to share with builders or family.
Design + Planning: £2,495
Our comprehensive package includes everything needed for planning submission and building regulations approval: architectural drawings, structural engineer calculations, planning application management, and unlimited revisions. You receive a complete, ready-to-build package.
Design-to-Build
For homeowners ready to proceed, our Design-to-Build service connects you with vetted local builders, provides project management, milestone payment protection, and a 12-month warranty. Our fee is transparent, plus a percentage of the build cost — so you only pay for what you use.
Frequently Asked Questions
How much does an extension cost per square metre in the UK?
In 2026, the average cost of a home extension in the UK is £1,800–£2,800 per square metre for a general single-storey rear extension. This varies by location, with London costing £2,340–£4,200/m² and the North of England costing £1,530–£2,380/m². Source: Checkatrade, FMB.
Is a single-storey or double-storey extension cheaper per m²?
Double-storey extensions are typically cheaper per square metre because the foundation and roof costs are spread over two floors. While the total cost is higher, the per-m² rate is usually 10–15% lower than a single-storey extension of the same footprint.
Do I need planning permission for an extension?
Many extensions fall under Permitted Development Rights, meaning no planning permission is required if they meet certain criteria (e.g., height limits, distance from boundaries, and volume limits). However, if you live in a conservation area, listed building, or your property has Article 4 restrictions, you will need full planning permission. The planning application fee is £206 for a householder application or £462 for a full application. Source: Planning Portal.
How much does an architect cost for an extension?
Architects typically charge 5–10% of the total build cost for a home extension. For a typical £75,000 extension, this equates to £3,750–£7,500. Some architects charge a fixed fee, particularly for smaller projects. Source: RICS fee guidelines.
What is the cheapest type of extension?
A garage conversion is the cheapest way to add habitable space, costing £800–£1,500 per m² (approximately £12,000–£22,500 for a typical single garage). The structure already exists, so you save on foundations, walls, and roof costs.
How long does an extension take to build?
A typical single-storey rear extension takes 10–14 weeks from breaking ground to completion. A double-storey extension takes 14–20 weeks. Loft conversions are usually quicker at 6–10 weeks. These timeframes exclude planning and design, which can add 2–6 months.
Can I get a mortgage or loan to fund an extension?
Yes. Most homeowners fund extensions through one of the following: remortgaging to release equity, a further advance on an existing mortgage, a personal secured loan, or home improvement loans. Speak to a mortgage broker to find the best option for your situation. The HomeOwners Alliance provides a useful guide to extension financing.
Does an extension add value to my home?
Yes. A well-designed extension typically adds 15–20% to your property's value, and in some cases up to 25%. The exact return depends on your location, the type of extension, and the quality of the finish. Read our full guide on extension value add for detailed analysis.
What is contingency, and do I need it?
Contingency is a budget buffer for unforeseen costs, typically 10–15% of the construction cost. You should always include it. Issues such as poor soil, hidden drainage, or asbestos discovery can increase costs significantly. Without contingency, you risk running out of funds mid-project.
Can I live in my house during an extension?
Yes, in most cases you can remain in your home during an extension, though it will be disruptive. Builders will need access through your property, and there will be noise, dust, and temporary loss of utilities. For large projects, some homeowners choose to move out for 4–8 weeks.
How do I get an accurate extension cost estimate?
The most accurate way is to use a tool that analyses real planning data from your area. Our free render tool shows you what has been approved nearby, what it cost, and what your property could achieve. This gives you a real-world benchmark rather than a generic estimate.
What is the difference between permitted development and planning permission?
Permitted Development (PD) allows certain types of work without a formal planning application, provided specific criteria are met. Planning permission requires a formal application to your local council with drawings and documents. PD is faster (no approval needed) but has strict limits on size, height, and location. Source: Planning Portal.
Should I hire a builder or a design-and-build company?
A design-and-build company handles everything from design to construction, which can reduce coordination headaches. Hiring an architect and builder separately gives you more control and potentially lower costs, but requires more project management. Our Design-to-Build service offers the best of both worlds: vetted builders, project management, and payment protection.
What is a party wall agreement, and do I need one?
A party wall agreement is required if your extension involves work on a wall shared with a neighbour, or if you are building within 3 metres of their wall. The cost ranges from £700 to £2,000 per neighbour, depending on whether they consent or dissent. Source: RICS party wall guidance.
Can I build an extension myself?
You can manage the project yourself and undertake non-structural work (e.g., demolition, decoration, basic landscaping). However, structural work, electrics, gas, plumbing, and building regulation sign-off must be completed by qualified professionals. Building control will inspect at key stages and issue a completion certificate only if work complies with regulations.
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